1. Field of the Invention
The present invention relates generally to cathode ray tube (CRT) displays and particularly to a circuit and method for generating electron beam deflection signals.
2. Description of the Prior Art
The image on a CRT is generated by using an electron beam to stimulate selected areas of a phosphorescent material located on the inside of the CRT screen. The scanning of the CRT face is accomplished by deflecting the electron beam relatively rapidly in one direction, usually horizontal, and relatively slowly in a second direction, usually vertical. Typically, the horizontal movement of the yoke is independent of vertical movement.
The phosphorescent material on the screen is continuous, but because of the manner in which the beam moves, the screen can be considered to consist of a large number of generally horizontal, parallel "raster lines" or lines of displayed information. As the beam scans along a raster line, the information about the level of stimulation to be given a particular area on the raster line is updated at fixed intervals in accordance with a clock pulse from the CRT timing unit. The electron beam normally performs 50 or 60 "frames" or complete scans of the CRT screen per second, depending on the external electrical power available. From the viewpoint of an observer facing the screen, the beam begins a frame at the left side of the top raster line of the CRT and moves generally horizontally along the line to the right side of the screen. The beam then performs a retrace to the left side of the next lower raster line and again begins to scan horizontally to the right. This continues until the beam reaches the right side of the lowest raster line, at which time a vertical retrace is performed during which the beam moves back to the beginning of the top raster line to begin the next frame.
The yoke, which causes the deflection of the video beam, is usually driven by horizontal and vertical deflection current signals. Because the CRT screen is substantially flat and because the beam is deflected through an angle, distortion will result in the CRT image if the deflection signals to the yoke are driven with a current ramp alone. The current signals to the yoke must be modified or adjusted such that the deflection rate of the beam varies in a manner that results in the beam moving along the CRT face at a substantially constant speed during a scan. Because the horizontal deflection is much more rapid than the vertical deflection, a different connection technique has usually been used for vertical movement than for horizontal. A common approach in the prior art for vertical adjustment has been the use of a sine wave as a correction function. This approach is fairly accurate, but, as implemented in the prior art, has certain drawbacks and disadvantages.
The prior art technique of generating the sine wave reguires the use of at least two adjustments, and in some designs three, to try to achieve proper height, frequency, and linearity of the sine wave. The need for these adjustments increases the complexity and cost of the CRT circuitry, as well as requiring the time and effort of the personnel who must attempt to "fine tune" the output. Also, in the prior art, the characters at the top and bottom of the screen cannot be made perfectly symmetrical and therefore will be of differing sizes. This can degrade the appearance of the CRT image.
The present invention relates to a novel circuit and method for generating the sine wave correction function which is free of the above noted problems.